BOSTON – The third win over the Toronto Maple Leafs was pretty difficult. And yet the Boston Bruins know that the fourth will be even harder.

The Bruins returned to the TD Garden for an optional practice yesterday, just hours after David Krejci completed a hat trick with an overtime winner that gave Boston a 3-1 lead over Toronto in the best-of-seven playoff series.

“I guess you see yourself already in the second round,” said Krejci, who scored the game-winner at 13:06 of the extra period. “But sometimes you forget about what you have to do. It’s really hard. It’s really tough to get the last win.”

Game 5 is tonight. With a win, the Bruins would advance to the Eastern Conference semifinals for the fourth time in five years and spoil Toronto’s hopes for its first playoff series victory since 2004.

“You know the other team is going to throw everything at you because it’s do-or-die for them,” said Bruins goalie Tuukka Rask, who stopped 45 shots in Game 4. “You kind of have to take the same mentality. If you slip even a little bit, if you give the other team life, then they’re going to take advantage of that. And I think that’s a challenge we’re facing too, you know. We have to approach the game like it’s a do-or-die game for us.”

The Leafs will try to come back without defenseman Mark Fraser, who took a puck to the face on Wednesday night and had surgery yesterday. Coach Randy Carlyle said Fraser was resting at home and had been texting and talking to his teammates yesterday morning.

“He’s a well-respected guy in our locker room,” captain Dion Phaneuf said. “He’s done a great job all year for us physically, playing big minutes. He’s just a great teammate. It was a scary situation but we’re glad he’s doing okay.”

John-Michael Liles is expected to take Fraser’s place in the lineup in Game 5.

Toronto is 2-12-1 in its last 15 games in Boston. The Bruins are 15-2 in series after taking a 3-1 lead, but they are only 9-8 in Game 5 with a chance to finish things off.

Toronto managed a split in Boston, winning Game 2, but then lost both games at home. On Wednesday night, the Leafs took a 2-0 lead in the first period and then needed less than a minute to tie it after the Bruins went up 3-2.

They outshot Boston 48-45, including a 25-16 edge in the third period and overtime. Matt Frattin hit the goalpost in overtime.

“We’ve done a lot of good things,” Carlyle said at the team’s training facility before flying to Boston. “You can’t change what happened last night. Now it’s time to regroup and that’s what we’re here to do. Gather this group together – reset, refocus, re-energize. Do all those things we normally do. We’ve got to go in and win a game in Boston.”

The Bruins won it when Phaneuf pinched up to try to stop Boston forward Nathan Horton and let the puck get by, setting up a two-on-one that Krejci used to beat James Reimer. Horton fell to the ice, apparently injured, but Coach Claude Julien said he was okay.

Phaneuf took responsibility for “a split-second decision.”

“Playoffs are about momentum and obviously that was a momentum swing,” he said. “But we’ve got a big game there on Friday. ... There’s no time to sit here and feel sorry about what happened.”

Only a handful of Toronto players practiced yesterday at the team’s training center, none of them regulars.

The Bruins also had an optional skate that was sparsely attended. Julien does not need to remind his team not to let up; they are just three years removed from losing the Eastern Conference semifinals after leading the series against Philadelphia 3-0.

“We know how hard it is to close. That’s the thing you hope your players realize extremely well after all the experiences we’ve had throughout the years,” he said. “We now know how hard it is to close and no reason for us to come out tomorrow and not play as hard, if not harder, than we did last night.”